Competent rope team experienced in trad including multi-pitch, but not climbed at this crag before. Grade of the route was within our joint capability, we had thoroughly researched the mechanics of ascent / descent. Got an early start and made good time on the 2 hour walk-in which was a familiar walking path to me. Weather was mild and dry. We were first to the crag, a pair of our friends climbed the route before us, crag was quiet until we heard a number of other groups arriving at once. In haste to get on the route and not hold up others, we didn't think through our rope setup and this led to a near-miss.

My partner led pitch 1 on a tri-rated rope as a single, I can't recall if this was a conscious decision (route is a straight line) or whether we just defaulted to tying into an end each, maybe out of greater familiarity (of using single rather than doubles) and partly to try and get started quickly / more simply. At some point before I took the lead for pitch 2, we realised we would likely need to switch to a double rope setup to facilitate the complex 'dropped loop' belay configuration on the summit. Safely secured to a belay on the ledge at the top of pitch 1, my partner untied from their end and tied into the middle. As the gear on pitch 2 was in a straight-enough line, I was happy to lead on one end, and my partner put me on belay on the end of the rope I had tied into on the ground (the live rope). Knowing that I 'only' needed the second end for dropping a loop to the pre-placed cam on the backside of the block AFTER summiting, I considered just clipping this second end to my haul line attachment point or a gear loop. But since it would later form part of the top belay, I decided it was much better to tie it into the regular harness points now (next to the live rope end) - I thought at this point I'd averted a near-miss, but actually this action contributed to one. Key factor: I'm tied into 2 ends, but only on belay on one.

We were slightly stressed at this moment, the wind had picked up and a party was just behind on the route, another on the ground and another on Needle Ridge overlooking us. I set off leading pitch 2 and whilst clipping a piece of gear, I confused the two rope ends (being the same colour) and clipped with the end I wasn't on-belay with! Thankfully I realised my mistake before moving up, unclipped it and clipped the live rope. Had I not noticed, moved up and fallen, I'd have taken a factor 2 fall onto the anchor, probably landed badly on the ledge below, and injured myself and the party waiting.